Plasma enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD) techniques are widely used for the development of a-Si layer or a-Si:H layer and different types of impurities into this a-Si or a-Si:H layer may be introduced by two processes, namely in-situ doping [15,[20], [21],[25], [26], [27], [28], [29], [30], [31], [32],[34], [35], [36], [37],39,44,45,47,59] (where different types of doping gases such as PH₃ for n poly-Si and B₂H₆ or B(CH)₃ for p poly-Si are added during the growth process) and ex-situ doping (ion-implantation) [60], [61], [62], [63]. High temperature annealing is then performed for the dopant activation and crystallization of this doped a-Si:H layer. Despite obtaining excellent result by using in-situ doping process, many researchers preferred ion-implantation technique for doping purpose of amorphous silicon layer to form TOPCon structure due to possessing many advantages such as precise dose control, ability of localized doping i.e. controllability of the penetration depth of the dopant atoms into the base material etc. [60], [61], [62], [63]. For an example, Feldmann et al. investigated the influential effect of using low energy ion implantation technique for the doping purpose of PECVD coated 20 nm thick undoped amorphous silicon layer [60]. In essence, the study was conducted to correlate the impact of ion energy and ion dose on the passivation quality of both phosphorous and boron doped TOPCon structures which were fabricated onto planar, <100> oriented, 200 μm, 1 Ω-cm, n-type FZ Si wafers. Besides, the samples were annealed at two different annealing temperatures (800°C and 900°C) for 60 min to activate the dopant and crystallize the doped amorphous silicon layer. Excellent surface passivation was obtained for phosphorous implantation with an implied V_oc (iV_oc) of greater than 720 mV at an annealing temperature of 800°C. Additionally, the samples which were annealed at 800°C showed little variations in iV_oc due to the change of ion doses. However, deterioration of iV_oc was observed with the increment of the ion doses for 900°C annealed samples. According to the authors, this inverse relationship was obtained due to the oxide's inability to inhibit the increment of phosphorous penetration from doped poly-Si layer into the bulk crystalline Si wafer at relatively higher annealing temperature of 900°C. As a result, Auger recombination (J_0,Auger) rate got increased within the diffused c-Si region near the c-Si/SiO_x interface. Besides, increment of surface recombination velocity (S_eff) at c-Si/SiO_x interface was also noticed which was also a strong reason for the decrement of the value of iV_oc at higher annealing temperature. On the other hand, contrary to n-contact, an enhancement of iV_oc was observed with the increment of ion doses for BF₂-implanted passivation contacts (p-contacts) at 800°C annealing temperature. However, poor passivation quality was obtained for the samples which were annealed at 900°C in spite of possessing low Auger recombination (J_0,Auger) value which meant diffusion of boron atoms into c-Si layer was not the cause for realizing poor iV_oc. Therefore, high carrier recombination rate within the doped poly-Si layer and at c-Si/SiO_x interface owing to the clustering of boron atoms might be a valid reason for obtaining such poor results. The authors finally recommended that the understandings may lead to simplify the development of IBC solar cells featuring passivation contacts. On the other hand, Reichel et al. studied the impact of P implantations into PECVD coated 15 nm boron doped a-Si layers [61]. After optimizing the ion doses, the Suns-V_oc samples demonstrated an outstanding iV_oc of greater than 700 mV (corresponding annealing temperature: 800°C) which in turn facilitated the development of IBC solar cells with ion implanted tunnel oxide passivated contacts based onto 1 Ω-cm, 200 μm thick, n-type FZ silicon wafers. The fabricated solar cells with an active area of 2 cm × 2 cm featured the highest V_oc of 682 mV and the highest pFF (pseudo fill factor) of 82.1%. Nevertheless, the authors pointed out that further investigations focusing on efficient charge carrier extraction are required for improving the cell performance. Furthermore, in another contribution, Reichel et al. claimed that for n-type passivated contacts, an excellent iV_oc of 725 mV was obtained by implanting P into undoped a-Si layers while slightly less iV_oc of 720 mV was realized when P implantations was made into in-situ B-doped a-Si layers [62]. In contrast, p-type passivated contacts were realized by either implanting BF₂ or B into intrinsic a-Si layers. BF₂ implantation provided excellent passivation quality with an iV_oc of 690 mV where as poor passivation quality was obtained for B implantations with an iV_oc of 640 mV. Improvement in the passivating quality by implanting BF₂ instead of B implantations may be attributed to the presence of F as it provides better interfacial passivation. However, a reduced FF was obtained for silicon solar cells featuring BF₂ implantations into undoped a-Si layers. Formation of non-ohmic Schottky contact may be responsible for this reduction in FF. On the other hand, the formation of this non-ohmic contact may be ascribed to: (i) the deactivation of B dopant atoms due to the formation of defect states at the surface of the poly silicon layer, and (ii) the increment of the effective thickness of SiO_x interfacial layer. Eventually, the both-side contacted solar cells featuring P implantations into undoped a-Si layers (to form electron contact) and in-situ B-doped hole contact demonstrated a V_oc of 690 mV and a FF of 79.1%. In contrast, the solar cells with P implantations into in-situ B doped a-Si layers featured a V_oc of 680 mV and a FF of 80.4%. The results and the understandings may smooth the progress of the fabrication of IBC solar cells with passivated contacts. Again, ion-implantation technique was used for the conversion of low pressure chemical vapor deposited (LPCVD) intrinsic a-Si layer into doped poly-Si layer for the formation of TOPCon structure [63]. In the contribution, the influential effect on the surface passivation properties of the ion implanted passivated contacts due to the variation of doping species, ion dose, and poly-Si thickness was investigated to facilitate the fabrication of both-sided contact solar cells with higher power conversion efficiency. The study revealed that superior surface passivation quality with very low recombination current density (J₀) of 4.5 fA/cm² was realized for n-type poly-Si contacts and the corresponding thickness of the poly-Si layer was 34 nm. On the other hand, significantly higher recombination current density (J₀) of 22 fA/cm² was obtained for p-type poly-Si contacts. The corresponding iV_oc value was 696 mV and the doped poly-Si layer was 34 nm thick. The other important observation was the realization of slight improvement in the passivation quality of p-type poly-Si contacts when BF was implanted for doping purpose instead of B. Finally, both sided poly-Si/SiO_x passivated contacts Si solar cells featuring n⁺ -poly-Si at the front side and p⁺ -poly-Si at the rear side with an active cell area of 2 cm × 2 cm were fabricated onto 250 μm thick, 1 Ω-cm, p-type FZ Si wafers. The best cell showed a V_oc of 709 mV and a FF of beyond 81%. But high sheet resistance for thinner poly-Si films and concurrently, limitations of achieving short-circuit current density (J_sc) of beyond 40 mA/cm² for thicker poly-Si films are one of the major concerns of these both sided poly-Si/SiO_x solar cells (in spite of featuring higher V_oc than standard Si heterojunction solar cells).

One may concern that use of low frequency industrial scalable direct plasma PECVD technique for depositing doped silicon layer onto ultra-thin SiO_x tunnelling layer may severely damage the oxide layer and hence the passivation quality will also be degraded with an immediate effect. However, excellent surface passivation quality with an implied V_oc (iV_oc) of above 735 mV and an implied FF (iFF) of 87.9% was realized when industrial scalable low frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique was used to deposit the doped amorphous silicon layer for the formation of TOPCon structure [64]. Besides, TLM (Transmission line measurement) technique revealed that the upper limit of the specific contact resistivities of the samples were in the range of 3–6 mΩ cm². Moreover, a lifetime test based on 100 Ω-cm, n-type FZ silicon wafers showed a very high minority carrier lifetime of 190 ms. Additionally, the study further revealed that better result was obtained when the low frequency PECVD coated doped poly-Si layer was combined with thermally grown ultra-thin interfacial tunnel oxide layer instead of combining with chemically grown oxide layer. On the other hand, Polzin et al. also obtained almost similar results by using low frequency batch-type direct plasma PECVD tool to realize doped Si layer for the formation of TOPCon structure [65]. Recently, another approach on the transformation of PECVD (13.56 MHz) coated phosphorous doped a-Si:H layer into n⁺ poly-crystalline Si layer by means of rapid thermal annealing (RTA) process was reported where several parameters such as annealing time (50–950 s), annealing temperature (780–900°C), cooling time (5–600 s) and the thickness of phosphorous doped a-Si:H layer (40–120 nm) were varied to facilitate the surface passsivation quality of n-type TOPCon structure [66]. The study revealed that RTA method took very low time of about 15 min for the whole crystallization process where as more than 60 min was needed in conventional tube furnace annealing process for the same purpose. Besides, no blistering during the thermal treatment was observed when the thickness of a-Si:H layer was less than 40 nm. But the passivation quality provided by RTA process with an iV_oc of 712 mV and J₀ of 12.5 fA/cm² was found to be slightly inferior to the passivation quality realized by using conventional tube furnace annealing process. However, a post-annealing treatment of hydrogenation with ALD coated Al₂O₃ capping layer resulted in a significant improvement of the surface passivation quality of RTA processed samples with an excellent iV_oc of 727 mV and a very low J₀ of 4.7 fA/cm². Eventually, the best cell with an active cell area of 2 cm × 2 cm demonstrated a power conversion efficiency of 23.04% with V_oc = 679 mV, J_sc = 41.97 mA/cm² and FF = 80.86%.

Atmospheric pressure chemical vapor deposition (APCVD) was found to be another suitable way to develop intrinsic or doped poly-Si layer [67,68]. The first report of APCVD coated doped poly-Si layer for the formation of POLO structure was presented by Markle et al. in the 35th European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC) which was held in 2018 [67]. According to that report, after performing high temperature thermal treatment and subsequent hydrogenation step adequate surface passivation quality was obtained for both n-type and p-type passivated contacts which featured chemically or thermally grown ultra-thin SiO_x layer and APCVD coated doped silicon layer. Quantitatively, the hydrogenated n-type and p-type POLO contacts based on CZ Si wafers provided quite low J₀ of 6 fA/cm² and acceptable iV_oc of 721 mV and 730 mV, respectively. The best p-POLO solar cell fabricated onto 156.75 mm × 156.75 mm, 1.6 Ω-cm, p-type CZ Si wafers offered a conversion efficiency of 19.5% with V_oc = 661 mV, J_sc = 37.7 mA/cm² and FF = 78.0%. The corresponding cell area was 92.7 cm². Recently, in 2022, Mousumi et al. claimed to realize a very low J₀ of 3 fA/cm² and an iV_oc of 712 mV for hydrogenated n-type passivated contacts in which the doped silicon layer was prepared by APCVD technique [68].

Physical vapour deposition (PVD) techniques such as sputtering were used for the development of doped silicon layer [69,70]. Yan et al. in their study developed in-situ boron doped silicon films to realize hole-selective passivated contacts by using a single co-sputtering system which comprised of undoped silicon (99.999% purity) and boron (99.999% purity) as target materials [69]. The stack of rf sputtered p-type silicon films and wet chemically grown ultra-thin SiO_x interlayer showed interesting passivation properties with a J₀ of less than 20 fA/cm² after undergoing a high temperature annealing step in N₂ atmosphere. Eventually, 2 cm × 2 cm p-TOPCon solar cells featuring sputter deposited doped Si layer were fabricated onto 200 μm, 1 Ω-cm, monocrystalline, p-type FZ Si wafers; the best conversion efficiency was found to be as high as 23.0% with V_oc = 701 mV, J_sc =41.1 mA/cm² and FF = 79.9%. On the other hand, David et al. developed phosphorous doped silicon layers onto ultra-thin SiO_x layer by using dc sputtering unit to form the n-type passivated contact [70]. Furthermore, symmetric lifetime samples based onto n-type c-Si wafers were fabricated and the highest value of iV_oc was found to be 695 mV whereas the median value was 680 mV. Therefore, from Ref. [69,70] we may conclude that both p-TOPCon and n-TOPCon structure may be developed with notable surface passivation quality by using simple sputtering process. Most importantly, use of toxic gases like silane, diborane and phosphine may be avoided if sputtering is used for the development of doped silicon layers. However, significant improvement in the pasivating properties of the TOPCon structure comprising of sputter deposited poly-Si layer is yet to be realized to compete with the state-of-the-art TOPCon structure featuring chemical vapour deposited poly-Si layer.

So far we have realized that high temperature annealing is a necessary and very important step to for the dopant activation and crystallization of the doped silicon layer. However, involvement of such high temperature annealing step undoubtedly increases the thermal budget during the fabrication of the solar cells. In this regard, several approaches have been initiated by the researchers to reduce the temperature of the required annealing process and/or find a suitable alternative method for fulfilling the purpose for which annealing is done [71,72]. For example, in 2018, Masuda et al. reported that reduction of crystallization temperature and time may be realized by using aluminium-induced crystallization (AIC) technique for the development of doped poly-Si layer to form p-TOPCon structure [71]. The authors further added that improvement in the carrier density in the p-type AIC poly-Si layer was realized by using B-doped Si target instead of a non-doped Si target during the deposition of a-Si layer. This high carrier density due to the successful incorporation of Al and B resulted in the reduction of the resistance of doped poly-Si layer. Eventually, enhancement in the power conversion efficiency with relatively higher V_oc and FF was noticed due to having lower series resistance and higher carrier density. However, further explorations are still required as the best cell featured a conversion efficiency of 13.5%. Recently, in 2020, Wilkes et al. claimed that the use of novel pulsed-laser processing method instead of high temperature annealing treatment for the dopant activation and crystallization of phosphorous doped a-Si:H layer in TOPCon solar cell structure provided excellent surface passivation quality [72]. Besides, an improvement in the surface passivation quality with an implied open-circuit voltage (iV_oc) of 718 mV, an effective minority carrier lifetime (τ) of 2 ms and single-sided recombination current density (J₀) of 8.6 mA/cm² was realized after depositing 60 nm SiN_x layer by PECVD method onto this laser processed TOPCon structure. However, the authors opined that further optimization of laser process and the thickness of a-Si:H layer may lead to maximize the interfacial passivation quality.